A critical issue for the neurobiology of drug addiction is what changes are responsible for the transition from non-dependent drug use to addiction. The goal of the present proposal from a new investigator is to begin to explore the neurobiological mechanisms responsible for drug escalation and define the molecular mechanisms responsible for mediating the change from drug use to addiction. To achieve this goal, we will test the following hypotheses: 1) Sufficient exposure to heroin and cocaine leads to changes in specific elements of the extended amygdala to produce elevations in hedonic set point that in turn leads to progressive elevation in drug intake. A subhypothesis is that the neurobiological basis of this transition to drug escalation reflects the development of drug addiction; 2) These neurobiological changes involve cellular effects at the translational and post translational levels that alter protein expression levels and function; 3) The transition to addiction does not occur as a result of one discrete neurochemical change; rather, a series of layered changes developing over the course of the drug escalation process; and 4) These changes represent the "switches" responsible for the transition from drug use to addiction, and are similar even for different classes of abused drugs. To test these hypotheses, we propose studies with the following Specific Aims: 1) To compare and contrast changes in protein expression and modification associated with excessive levels of heroin intake with those associated with escalated cocaine intake; 2) To define the time course of changes in protein expression and modification during the development of heroin and cocaine escalation; and 3) To define persistent changes in protein expression and modification associated with abstinence and with relapse upon re-exposure to heroin or cocaine. Our proposed combination of cutting-edge behavioral, neuroanatomical, and proteomic approaches will permit the identification of important molecular substrates of the transition from casual drug use to addiction. It is hoped that these findings will eventually lead to improved treatments for these devastating conditions.